Production of lightweight polybenzimidazole insulative material

ABSTRACT

A process is provided for forming a lightweight polybenzimidazole product which is both high temperature resistant and flameproof. A plurality of adjoining segments of hollow polybenzimidazole fiber are contacted with a solvent for the same, the solvent is evaporated thereby bonding the adjoining segments at locations where contact is made, and the resulting lightweight product is heated at an elevated temperature until volatile components present within the fibrous product are substantially removed. The resulting product is particularly useful as an insulative barrier in aerospace applications.

United States Patent 1191 Dunay Nov. 27, 1973 1 PRODUCTION OFLIGHTWEIGHT POLYBENZIMIDAZOLE INSULATIVE MATERIAL [75] Inventor: [73]Assignee: Celanese Corporation, New York,

Michael Dunay, Fanway, NJ.

156/305, 306, 307, 244, 314, 180; 18/8 SC, 8 QM, 8 OD; 28/73 [56]References Cited OTHER PUBLICATIONS Polymer Resists Heat, Chem. and Eng.News, Vol. 38,

No. 48, Nov. 28, 1960, p. 41. New High Temperature Polymers: OrderedHeterocycle Copolymers, Preston et a1., Amer. Chem. Soc.,

Primary Examiner-Carl D. Quarforth Assistant Examiner-P. A. NelsonAttorney-Thomas J. Morgan, Charles B. Harris and Kenneth E. Macklin [57]ABSTRACT A process is provided for forming a lightweightpolybenzimidazole product which is both high temperature resistant andflameproof. A plurality of adjoining segments of hollowpolybenzimidazole fiber are contacted with a solvent for the same, thesolvent is evap- UNITED STATES PATENTS orated thereby bonding theadjoining segments at 10- 3,142,101 9/1964 Allman, Jr. et a1. 156/306 xcations Where Contact is made, and the resulting g 3,179,630 4/1965Endrey 161/189 X weight product is heated at an elevated temperature3,271,220 /1 66 G e /239 X until volatile components present within thefibrous 3,352,734 1 H1967 f y et 156/305 X product are substantiallyremoved. The resulting prod- 3,365,354 BI'IIIOII X t i rti ularly usefulas an insulative barrier in 3,516,900 6/1970 Mallonee et a1. 161/150aerospace applications 3,573,130 3/1971 Vinton et a1 156/305 X 3,574,5234/1971 Hudson et a1 161/150 X 16 Claims, 1 Drawing Figure MPfi- -1 lfifz PRODUCTION OF LIGHTWEIGHT POLYBENZIMIDAZOLE INSULATIVE MATERIALBACKGROUND OF THE INVENTION In recent years the search for lightweightheat resistant and flameproof insulating materials has been intensifiedbecause of demands for such products presented by the aerospaceindustry.

Those interested in the evaluation of the various insulative productswhich have been proposed are becoming increasingly aware of theimportance of the total fire hazard concept. This concept takes intoconsideration whether the product is apt to evolve smoke or other toxicvapors when subjected to flame or elevated temperatures such as would beencountered upon the combustion of nearby flammable materials. Forinstance, it is generally recognized that in many use areas thepotential evolution of toxic gases by an insulating material upon theapplication of heat may be a hazard at least as great as its potentialflammability.

It is an object of the invention to provide a process for the productionof a lightweight product which is particularly useful as an insulativebarrier in aerospace applications.

It is another object of the invention to provide a lightweight productwhich is flameproof.

It is a further object of the invention to provide a lightweight productwhich is capable of withstanding elevated temperatures without theevolution of toxic gases.

These and other objects, as well as the scope, nature, and utilizationof the invention will be apparent from the following detaileddescription, and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for forming ahigh temperature resistant product comprises:

a. providing a plurality of adjoining segments of hollowpolybenzimidazole fiber,

b. contacting the adjoining segments of hollow polybenzimidazole fiberwith a solvent for the polybenzimidazole in a quantity sufficient tosolubilize a portion of the fiber surface while maintaining the originalhollow fibrous configuration of the segments essentially intact,

c. evaporating a quantity of the solvent thereby bonding the adjoiningsegments of hollow polybenzimidazole fiber at points of contact, and

d. heating the resulting bonded product to a temperature of at least400C. where it is maintained until volatile components present thereinare substantially removed.

The resulting lightweight product may be advantageously utilized as aninsulative barrier in aerospace applications, as well as in othersimilar use areas.

DESCRIPTION OF THE DRAWING The drawing illustrates a portion of aninsulative panel formed in accordance with the present invention, Thepanel illustrated comprises a plurality of short segments of hollowpolybenzimidazole fibers which are bonded together at their points ofcontact.

DESCRIPTION OF PREFERRED EMBODIMENTS The polymeric material utilized toform the hollow fiber is a linear polybenzimidazole. Typical polymers ofthis class and their preparation are more fully described in US. Pat.No. 2,895,948, US. Reissue Pat. No. 26,065, and in the Journal ofPolymer Science, Vol. 50, pages 511-539 (1961) which are hereinincorporated by reference. The polybenzimidazoles consist essentially ofrecurring units of the following Formulas I and II. Formula I is:

wherein R is a tetravalent aromatic nucleus, preferably symmetricallysubstituted, with the nitrogen atoms forming the benzimidazole ringsbeing paired upon adjacent carbon atoms, i.e., ortho carbon atoms, ofthe aromatic nucleus, and R is a member of the class consisting of (1)an aromatic ring, (2) an alkylene group (preferably those having four toeight carbon atoms), and (3) a heterocyclic ring from the classconsisting of (a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e)N thiophene, and (f) pyran. Formula II is:

wherein Z is an aromatic nucleus having the nitrogen atoms forming thebenzimidazole ring paired upon adjacent carbon atoms of the aromaticnucleus.

Preferably, aromatic polybenzimidazoles are selected, e.g., frompolymers consisting essentially of the recurring units of Formulas I andII wherein R' is an aromatic ring or a heterocyclic ring.

As set forth in US. Reissue Pat. No. 26,065, the aromaticpolybenzimidazoles having the recurring units of Formula II may beprepared by self-condensing a trifunctional aromatic compound containingonly a single set of ortho disposed diamino substituents and anaromatic, preferably phenyl, carboxylate ester substituent. Exemplary ofpolymers of this type is poly-25(6)- benzimidazole prepared by theautocondensation of phenyl-3 ,4-diaminobenzoate.

As also set forth in the above-mentioned patent, the aromaticpolybenzimidazoles having the recurring units of Formula I may beprepared by condensing an aromatic tetraamine compound containing a pairof orthodiamino substituents on the aromatic nucleus with a dicarboxylcompound selected from the class consisting of (a) the diphenyl ester ofan aromatic dicarboxylic acid, (b) the diphenyl ester of a heterocyclicdicarboxylic acid wherein the carboxyl groups are substituents uponcarbon in a ring compound selected from thepoly-2,2-(biphenylene-4",4)-5,5-

bibenzimidazole; poly-2,2 '-amylene-5 ,5 '-bibenzimidazole;poly-2,2'-octarnethylene-5,5 '-bibenzimidazole; poly-2,6-(m-phenylene)-diimidazobenzene; poly-2,2 '-cyclohexeneyl-5 ,5-bibenzimidazole; poly-2,2 m-phenylene )-5 ,5 -di(benzimidazole) ether;poly-2,2'(m-phenylene)-5,5'-di(benzimidazole) sulfide; poly-2,2m-phenylene )-5 ,5 -di( benzimidazole) sulfone;poly-2,2(m-phenylene)-5,5'-di(benzimidazole) methane; poly-2 ',2'm-phenylene )-5 ',5 'di(benzimidazole) propane-2,2; and poly-2', 2m-phenylene)-5 ,5 di( benzimidazole) ethylene-1,2 where the double bondsof the ethylene groups are intact in the final polymer. I

The preferred polybenzimidazole for use in the present process is oneprepared from poly-2,2-(mphenylene)-5,5'-bibenzimidazole, the recurringunit of which is:

Any polymerization process known to those skilled in the art may beemployed to prepare the polybenzimidazole. With respect to aromaticpolybenzimidazoles, preferably, equimolar quantities of the monomerictetraamine and dicarboxyl compound may be introduced into a first stagemelt polymerization reaction zone and heated therein at a temperatureabove about 200C., preferably at least 250C., and more preferably fromabout 270 to 300C. The reaction is conducted in a substantiallyoxygen-free atmosphere, i.e., below about 20 ppm oxygen and preferablybelow about 8 ppm oxygen, until a foamed prepolymer is formed. Usually,the first stage reaction is continued until a prepolymer is formedhaving an inherent viscosity, expressed as deciliters per gram, of atleast 0.1, and preferably from about 0.13 to 0.3 (determined from asolution of 0.4 grams of the polymer in 100 ml. of 97 per cent H 80 at25C.).

After the conclusion of the first stage reaction, which normally takesat least 0.5 hour and preferably 1 to 3 hours, the foamed prepolymer iscooled and then powdered or pulverized in any convenient manner. Theresulting prepolymer powder is then introduced into a second stagepolymerization reaction zone wherein it is heated under substantiallyoxygen-free conditions, as described above, to yield a polybenzimidazolepolymer product, desirably having an I.V., as measured above, of atleast 0.4, e.g., 0.8 to 1.1 or more. The temperature employed in thesecond stage is at least 250C, preferably at least 325C., and morepreferably from about 350 to 425C. The second stage reaction generallytakes at least 0.5 hours, and preferably from about 1 to 4 hours ormore.

Hollow polybenzimidazole fiber for use in the present process may beformed by any one of a variety of techniques as will be apparent tothose skilled in the art. For instance, dry or wet spinning techniquesmay be utilized to form the hollow fiber utilizing an extrusion orificeof a suitable configuration to form a tubular fiber. Alternatively, thehollow fiber utilized in the present process may be formed in accordancewith the teachings of commonly assigned U.S. Pat. No. 3,494,121 ofThomas C. Bohrer which is herein incorporated by reference.

In a preferred embodiment of the invention the hollow polybenzimidazolefiber is formed by (a) providing a spinning solution of a fiber-formingpolybenzimidazole dissolved in a spinning solvent selected from thegroup consisting of N,N-dimethyl formamide, N,N- dimethyl acetamide,dimethyl sulfoxide, and N-methyl- 2-pyrrolidone, (b) extruding thesolution into a coagulation bath which is a non-solvent for thepolybenzimidazole and capable of coagulating the periphery of theresulting polybenzimidazole fiber, (c) removing the resultingpolybenzimidazole fiber from the coagulation bath following thecoagulation of the periphery of the fiber while the fiber continues tomaintain a substantial quantity of the solvent within the interiorthereof, and (d) passing the fiber containing a substantial quantity ofthe solvent within the interior thereof through a heating zone capableof promoting the expansion thereof to form a hollow polybenzimidazolefiber.

The preferred spinning solvent for use in forming the spinning solutionis N,N-dimethyl acetamide. The spinning solution may be prepared bydissolving sufficient polybenzimidazole in the solvent (i.e.,N,N-dimethyl formamide, N,N-dimethyl acetamide, dimethyl sulfoxide, orN-methyl-2-pyrrolidone) to yield a final solution suitable for extrusioncontaining about 20 to 30 per cent by weight of polymer, based upon thetotal weight of the solution, and preferably from about 20 to 24 percent by weight polymer. The viscosity of the spinning solution should bewithin the range of 500 to 8000 poises measured at 30C., and preferablyin the range of about 1,500 to 4,000 poises at 30C.

One suitable means for dissolving the polymer in the solvent is bymixing the materials at a temperature above the normal boiling point ofthe solvent, for example, about 25 to C. above such boiling point, andat a pressure of 2 to 15 atmospheres for a period of l to 5 hours. Theresulting solutions are then filtered to remove any undissolved polymerwhich would otherwise clog the spinning orifice. A minor amount oflithium chloride optionally may be provided in the spinning solution inaccordance with the teachings of commonly assigned U.S. Pat. No.3,502,606 of Anthony B. Conciatori and Charles L. Smart which is hereinincorporated by reference.

The spinning solution is extruded into a coagulation bath which is anon-solvent for the polybenzimidazole and capable of coagulating theperiphery of the resulting fiber. While passing through the coagulationbath, an outer skin or layer of precipitated polymer is formed upon thesurface of the resulting polybenzimidazole fiber. A substantial quantityof the polybenzimidazole spinning solvent is retained within theinterior of the resulting fiber, and the interior of the resulting fiberis of a substantially lesser density than the coagulated peripherythereof.

Any coagulation bath capable of coagulating the fiber surface in arelatively rapid fashion may be selected for use in the process. Aqueouscogaulation baths of N,N-dimethyl formamide, N,N-di rnethyl acetamide,dimethyl sulfoxide or N-methyl-Z-pyrrolidone may be selected. Suchaqueous coagulation baths preferably contain the same solvent asemployed in the spinning solution and at least about 50 per cent waterby weight (e.g. about 50 to 65 per cent water by weight). The preferredcoagulation bath for use in the process is tetrachloroethylene. Otherillustrative coagulation baths include methylene chloride, ethylenedicloride, methyl chloroform, trichloroethylene, and aliphatichydrocarbons such as hexane, cyclohexane, and heptane. The coagulationbath is preferably circulated and preferably flows inthe same directionas the extruded fiber.

The temperature of the spinning solution during the extrusion stepshould be within the range of about C. up to just below the boilingpoint of the spinning solvent. The temperature of the coagulation bathshould be within the range of about 10C. up to just below the boilingpoint of the coagulation bath. The spinning solution and the coagulationmay be each conveniently provided at about room temperature (i.e. aboutC.). Conventional circular extrusion orifices may be utilized. Theresulting as-spun fibers commonly have a denier of about 10 to 200, ormore. The residence time of the as-spun fiber within the coagulationbath should be such that the periphery of the fiber is coagulated whilethe interior thereof tends to be unprecipitated and continues to retaina substantial quantity of the spinning solvent. In a preferredembodiment of the invention the extruded fiber experiences a residencetime of about 1 to 60 seconds within the coagulation bath. Extrusionspeeds of l to 10 meters per minute, or more, may be selected.

The resulting polybenzimidazole fiber following its removal from thecoagulation bath is next passed through a heating zone capable ofpromoting the expansion thereof to a hollow polybenzimidazole fiber. Ina preferred embodiment of the invention the fiber following its removalfrom the coagulation bath is continuously passed in the direction of itslength to the heating zone wherein the solvent present within theinterior thereof causes the fiber to expand to a tubular configurationhaving a continuous hollow core. No intermediate wash step isrecommended since the presence of a substantial quantity of solventwithin the interior of the surface coagulated fiber is essential tobring about the expansion thereof to the desired hollow configuration.

The heating zone utilized to bring about the expanheated contact surfaceover which the fiber is continuously passed, or a heated gaseousatmosphere. In a preferred embodiment of the invention the heating zonecomprises a hot shoe over which the surface coagulated fiber is passedwhile in contact with the same. The heating zone is preferably providedat a temperature ranging from the boiling point of the spinning solventup to about 300C. (e.g. 200 to 300C). In a particularly preferredembodiment of the process the heating zone is a heated contact surfaceprovided at a terriperature of about 200 to 225C.

The residence time for the fiber within the heating zone is relativelybrief and is adjusted so that the requisite expansion to form a tubularfiber occurs in the absence of appreciable thermally induced molecularcross-linking which renders the polybenzimidazole insoluble in many ofits common solvents, such as N,N- dimethyl acetamide. Residence timeswithin the heating zone commonly range from 1 to 60 seconds, andpreferably from 10 to 60 seconds. If desired, the tensile properties ofthe resulting hollow fiber may be optionally enhanced by placing thesame under a longitudinal tension sufficient to draw the same whilepassing through the heat treatment zone. Alternatively, such optionaldrawing may be conducted in a separate hot drawing zone as will beapparent to those skilled in the art in accordance with conventionalfiber drawing techniques. v 1

The hollow polybenzimidazole fiber regardless of the technique used inits formation is placed in the configuration desired for the finalproduct of the present process. The fiber is positioned in such a mannerthat a plurality of adjoining segments of the hollow fiber are incontact. In a preferred embodiment of the invention the hollowpolybenzimidazole fiber is provided as a random array of fiber segmentshaving lengths of about 0.1 to 1 inch which are contained within a frameor support having a configuration identical to that of the desiredproduct. Alternatively, such relatively short fiber segments may beprovided in an oriented configuration. If desired, adjoining segments ofthe hollow fiber may be provided through the winding of a continuouslength of the same upon a support.

The adjoining segments of hollow polybenzimidazole fiber are nextcontacted with a bonding solvent for the polybenzimidazole in a quantitysufficient to solubilize a portion of the fiber surface whilemaintaining their original hollow fibrous configuration essentiallyintact. The bonding solvent selected may be identical to the spinningsolvent (i.e. N,N-dimethyl formamide, N,N- dimethyl acetamide, dimethylsulfoxide, or N-methyl- 2-pyrrolidone). Alternatively, the bondingsolvent may be selected from other solvents which are commonlyrecognized in the art to be solvents for a polybenzimidazole. Forinstance, other illustrative bonding solvents include formic acid andsulfuric acid. Contact between the hollow polybenzimidazole fiber andbonding solvent may be conveniently accomplished by spraying the fiberwith the bonding solvent so that the solvent impinges upon and coats thesurface of the hollow fiber. Alternatively the bonding solvent may beallowed to briefly flow through an array or mat of the adjoiningsegments of hollow fiber wherein the fiber surface is coated with thebonding solvent.

In a further embodiment of the process a continuous length of the hollowpolybenzimidazole fiber while wound upon a support may be brieflyimmersed in a vessel containing the bonding solvent wherein the fibersurface becomes coated upon its removal. The bonding solventdnayoptionally include a quantity of polybenzimidazole dissolved thereinprior to its contact with the hollow fiber. I

A quantity of the bpnding solvent while in contact with the adjoiningsegments of hollow polybenzimidazole fiber is next evaporated toaccomplish bonding of the fiber segments at their points of contact.Evaporation of the solvent is preferably largely conducted at atemperature ranging from the boiling point of the solvent up to about C.in excess of the boiling point of the solvent (e.g. at about 200 to300C).

As the solvent is evaporated, adhesion between adjoining fiber segmentsoccurs.

By conducting the evaporation of the bonding solvent at or near itsboiling point a permanent bond between adjoining fiber segments occurswithout the violent evolution of bonding solvent or the rearrangement ofadjoining fiber segments. If desired, the fiber while in contact withthe bonding solvent may be heated at a constant temperature whileevaporation occurs, or the temperature may be gradually elevated (e.g.from about 200 to 400C.). Suitable times during which a quantity of thebonding solvent is evaporated toaccomplish bonding of the hollow fibersegments commonly range from about 1 to 60 minutes. Longer bonding timesmay be employed but generally yield no commensurate advantage. Theapparatus in which bonding is conducted may be varied, and mayconveniently be a circulating air oven.

Following the bonding of adjoining fiber segments the bonded product isnext heated at a temperature of at least 400C. (e.g. 400 to 700C.), andpreferably at a temperature of at least 500C. (e.g. 500 to 700C.) whereit is maintained until residual volatile components present therein aresubstantially removed. The final heating step of the process may beconducted for about 2 to 15 minutes, or more. During this final thermaltreatment residual volatile components (e.g., residual spinning andbonding solvent) present therein are substantially evolved therebyeliminating the hazard presented by the possible volatilization of suchcomponents should the product ever be subjected to highly elevatedtemperatures during utilization. Also the final thermal treatmentpromotes cross-linking of molecules within the bonded polybenzimidazoleproduct which renders the same insoluble in common polybenzimidazolesolvents, such as N,N-dimethyl acetamide. The final heat treatment stepis preferably conducted in an oxygen-containing atmosphere, such asordinary air, wherein the cross-linking of molecules includes oxidativecross-linking.

The product of the present invention is black in appearance,lightweight, and high temperature resistant. The product is flameproofand will not burn when placed in the flame of a microbumer.

The resulting high temperature resistant product is particularly usefulas an insulative material which finds particular utility in theaerospace industry or in similar applications where its uniqueproperites may be put to good use. For instance, self-supportinginsulative panels may be formed. If desired, the product may be cut tovarious configurations required for specific end uses following itsformation.

The following example is given as a specific illustration of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the example.

EXAMPLE A spinning solution of a fiber-forming polybenzimidazole havinga Brookfield low shear viscosity of 2,200 poises at 30C. is preparedemploying N,N- dimethyl acetamide as spinning solvent containing 24 percent by weight of poly-2,2'-(m-phenylene)-5,5'- bibenzimidazole basedupon the total weight of the solution, and 2 per cent by weight oflithium chloride based upon the total weight of the solution. Thedissolution of the polymer is accomplished by agitating the same whilein particulate form with the N,N-dimethyl acetamide solvent (in whichlithium chloride was previously dissolved) while in a closed zone at atemperature of about 230C. The resulting solution is filtered to removeany residual solids.

The spinning solution while at a temperature of 25C. is fed to spinnerethaving a single hole of 316 microns diameter and is extruded at a rateof 5 meters per minute into a circulating coagulation bath oftetrachloroethylene having a temperature of 25C. which flows in thedirection of the as-spun fiber. The resulting as-spun fiber is retainedwithin the coagulation bath for a residence time of 15 seconds duringwhich time a coagulated skin is formed upon the surface of the fiberwhile i the core of the fiber is not coagulated and continues to retaina substantial quantity of the N,N-dimethyl acetamide solvent.

The resulting as-spun polybenzimidazole fiber is passed directly to ahot shoe provided at a temperature of 215C. over which it is passedwhile in contact with the same for a residence time of 15 seconds. Whilein contact with the hot shoe, the as-spun polybenzimidazole fiberexpands to a hollow tubular configuration.

The tubular fiber is chopped into a plurality of lengths having alongest dimension of about 0.5 inch. The lengths are provided in arandom array on a support having upright edges. The configurationdefined by the support and its upright edges corresponds to theconfiguration of the rectangular insulating panel which is ultimately tobe produced.

The arrayor mat of tubular polybenzimidazole fibers is sprayed with aN,N-dimethyl acetamide bonding solvent provided at 25C. The bondingsolvent impinges upon and coats the fiber surfaces. The outer surfaceonly of the polybenzimidazole fiber is solubilized by the bondingsolvent. Excess bonding solvent is withdrawn as it collects upon thebase of the support through openings provided therein.

The support together with the solvent coated fiber mat is next placed ina circulating air oven provided at 200C. A substantial quantity of thebonding solvent volatilizes and is removed from the oven and the bondingof adjoining segments of polybenzimidazole fiber at point of contactoccurs. The circulating air oven is maintained at 200C. for 10 minutes,and is then gradually raised to 400C. over a period of one hour.

The bonded product is next heated in a circulating air oven provided at500C. for 10 minutes during which time residual traces of volatilecomponents such as the N,N-dimethyl acetamide spinning and bondingsolvents are substantially expelled.

The resulting product is removed from the support and may be directlyutilized as an insulative panel. A portion of the panel is,illustratedin the drawing. The panel comprises a plurality of relatively shorthollow segments of polybenzimidazole fiber which are bonded together attheir points of contact. The panel is black in appearance, lightweight,high temperature resistant, and non-buming when heated in the flame of amicrobumer.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations andmodifications are considered within the purview and scope of the claimsappended hereto.

I claim:

1. A process for forming a high temperature resistant productcomprising:

a. providing a plurality of adjoining segments of hollowpolybenzimidazole fiber,

b. contacting said adjoining segments of hollow polybenzimidazole fiberwith a solvent for said polybenzimidazole in a quantity sufficient tosolubilize a portion of the fiber surface while maintaining the originalhollow fibrous configuration of said segments essentially intact,

evaporating a quantity of said solvent thereby bonding said adjoiningsegments of hollow polybenzimidazole fiber at points of contact, andheating the resulting bonded product to a temperature of at least 400C.where it is maintained until volatile components thereof aresubstantially removed and cross-linking of molecules within the bondedpolybenzimidazole product is promoted which renders the same insolublein common polybenzimidazole solvents.

2. A process according to claim 1 wherein said polybenzimidazoleconsists essentially of recurring units of the formula:

wherein R is a tetravelent aromatic nucleus, with the nitrogen atomsforming the benzimidazole rings paired upon adjacent carbon atoms ofsaid aromatic nucleus, and R is selected from the group consisting of(1) an aromatic ring, (2) an alkylene group having from four to eightcarbon atoms, and (3) a heterocyclic ring selected from the groupconsisting of (a) pyridine, (b) pyrazine, (c) furan, (d) quinoline, (e)thiophene, and (f) pyran.

3. A process according to claim 1 wherein said polybenzimidazole ispoly-2,2 -(m-phenylene )-5 ,5 bibenzimidazole.

4. A process according to claim 1 wherein said solvent for saidpolybenzimidazole is selected from the group consisting of N,N-dimethylformamide, N,N- dimethyl acetamide, dimethyl sulfoxide and N-methyl-2-pyrrolidone.

, 5. A process according to claim 4 wherein said solvent is N,N-dimethylactamide.

6. A process according to claim 1 wherein step (c) is conducted byheating said adjoining segments of hollow polybenzimidazole fiber whilein contact with said solvent for said polybenzimidazole at a temperatureof about 200 to 300C.

7. A process according to claim 1 wherein said resulting bonded productis heated in an oxygen-containing atmosphere to a temperature of atleast 400C.

8. A process according to claim 1 wherein said resulting bonded productis heated in an oxygen-containing atmosphere to a temperature of atleast 500 C.

9. A process for forming a high temperature resistant productcomprising:

a. providing a spinning solution of a fiber-forming polybenzimidazoledissolved in a spinning solvent selected from the group consisting ofN,N-dimethyl formamide, N,N-dimethyl acetamide, dimethyl sulfoxide, andN-methyl-2-pyrrolidone,

b. extruding said solution into a coagulation bath which is anon-solvent for said polybenzimidazole and capable of coagulating theperiphery of the resulting polybenzimidazole fiber,

c. removing said resulting polybenzimidazole fiber from said coagulationbath following the coagulation of the periphery of said fiber while saidfiber continues to maintain a substantial quantity of said solventwithin the interior thereof,

d. passing said fiber containing a substantial quantity of said solventwithin the interior thereof through a heating zone capable of promotingthe expansion thereof to form a hollow polybenzimidazole fiber,

e. providing a plurality of adjoining segments of said hollowpolybenzimidazole fiber,

f. contacting said adjoining segments of hollow polybenzimidazole fiberwith a bonding solvent for said polybenzimidazole provided in a quantitysufficient to solubilize a portion of the fiber surface whilemaintaining the original hollow fibrous configuration of said segmentsessentially intact,

. evaporating said bonding solvent thereby bonding said adjoiningsegments of hollow polybenzimidazole fiber at points of contact, andheating theresulting bonded product to a temperature of at least 400C.where it is maintained until volatile componentspresent therein aresubstantially removed and cross-linking of molecules within the bondedpolybenzimidazole product is promoted which renders the same insolublein common polybenzimidazole solvents.

10. A process according to claim 9 wherein said polybenzimidazoleconsists essentially of recurring units of the formula:

wherein R is a tetravalent aromatic nucleus, with the nitrogen atomsforming the benzimidazole rings paired upon adjacent carbon atoms ofsaid aromatic nucleus, and R is selected from the group consisting of(1) an aromatic ring, (2) an alkylene group having from four to eightcarbon atoms, and (3) a heterocyclic ring selected from the groupconsisting of (a) pyridine, (b) pyrazine, (c) fuma, (d) quinoline, (e)thiophene, and py 11. A process according to claim 9 wherein saidpolybenzimidazole is poly-2,2-(m-phenylene )-5 ,5 bibenzimidazole.

12. A process according to claim 9 wherein said spinning solvent isN,N-dimethyl acetamide.

13. A process according to claim 9 wherein said plurality of adjoiningsegments of said hollow polybenzimidazole fiber comprise a mat of hollowfibers having lengths of about 0.1 to 1 inch.

14. A process according to claim 9 wherein said bonding solvent isselected from the group consisting of N,N-dimethyl formarnide,N,N-dimethyl acetamide, dimethyl sulfoxide, and N-methyl-Z-pyrrolidone.

15. A process according to claim 9 wherein said resulting bonded productis heated in an oxygencontaining atmosphere to a temperature of at least400C.

16. A process according to claim 9 wherein said resulting bonded productis heated in an oxygencontaining atmosphere to a temperature of at least500C.

2. A process according to claim 1 wherein said polybenzimidazoleconsists essentially of recurring units of the formula:
 3. A processaccording to claim 1 wherein said polybenzimidazole ispoly-2,2''-(m-phenylene)-5,5''-bibenzimidazole.
 4. A process accordingto claim 1 wherein said solvent for said polybenzimidazole is selectedfrom the group consisting of N,N-dimethyl formamide, N,N-dimethylacetamide, dimethyl sulfoxide and N-methyl-2-pyrrolidone.
 5. A processaccording to claim 4 wherein said solvent is N,N-dimethyl actamide.
 6. Aprocess according to claim 1 wherein step (c) is conducted by heatingsaid adjoining segments of hollow polybenzimidazole fiber while incontact with said solvent for said polybenzimidazole at a temperature ofabout 200* to 300*C.
 7. A process according to claim 1 wherein saidresulting bonded product is heated in an oxygen-containing atmosphere toa temperature of at least 400*C.
 8. A process according to claim 1wherein said resulting bonded product is heated in an oxygen-containingatmosphere to a temperature of at least 500* C.
 9. A process for forminga high temperature resistant product comprising: a. providing a spinningsolution of a fiber-forming polybenzimidazole dissolved in a spinningsolvent selected from the group consisting of N,N-dimethyl formamide,N,N-dimethyl acetamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone,b. extruding said solution into a coagulation bath which is anon-solvent for said polybenzimidazole and capable of coagulating theperiphery of the resulting polybenzimidazole fiber, c. removing saidresulting polybenzimidazole fiber from said coagulation bath followingthe coagulation of the periphery of said fiber while said fibercontinues to maintain a substantial quantity of said solvent within theinterior thereof, d. passing said fiber containing a substantialquantity of said solvent within the interior thereof through a heatingzone capable of promoting the expansion thereof to form a hollowpolybenzimidazole fiber, e. providing a plurality of adjoining segmentsof said hollow polybenzimidazole fiber, f. contactiNg said adjoiningsegments of hollow polybenzimidazole fiber with a bonding solvent forsaid polybenzimidazole provided in a quantity sufficient to solubilize aportion of the fiber surface while maintaining the original hollowfibrous configuration of said segments essentially intact, g.evaporating said bonding solvent thereby bonding said adjoining segmentsof hollow polybenzimidazole fiber at points of contact, and h. heatingthe resulting bonded product to a temperature of at least 400*C. whereit is maintained until volatile components present therein aresubstantially removed and cross-linking of molecules within the bondedpolybenzimidazole product is promoted which renders the same insolublein common polybenzimidazole solvents.
 10. A process according to claim 9wherein said polybenzimidazole consists essentially of recurring unitsof the formula:
 11. A process according to claim 9 wherein saidpolybenzimidazole is poly-2,2''-(m-phenylene)-5,5''-bibenzimidazole. 12.A process according to claim 9 wherein said spinning solvent isN,N-dimethyl acetamide.
 13. A process according to claim 9 wherein saidplurality of adjoining segments of said hollow polybenzimidazole fibercomprise a mat of hollow fibers having lengths of about 0.1 to 1 inch.14. A process according to claim 9 wherein said bonding solvent isselected from the group consisting of N,N-dimethyl formamide,N,N-dimethyl acetamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone.15. A process according to claim 9 wherein said resulting bonded productis heated in an oxygen-containing atmosphere to a temperature of atleast 400*C.
 16. A process according to claim 9 wherein said resultingbonded product is heated in an oxygen-containing atmosphere to atemperature of at least 500*C.